1) Field of the Invention
The present invention relates to a process for preparing catalysts comprising metals from groups VI and VIII on a support, as well as to the use of the said catalysts for the water-treatment of hydrocarbon charges.
2) Background Art
A standard process for manufacturing water-treatment catalysts containing metals from groups VI and VIII consists in impregnating, for example by immersion, spraying or mixing, a porous inorganic support with a common solution or individual solutions of soluble compounds of metals from groups VI and VIII, working in one or more steps. After impregnation, the support thus treated is dried and can undergo various treatments, for example heating in an oxidative, neutral or reductive atmosphere, sulphurization with hydrogen sulphide or with other sulphur compounds, in particular organosulphur compounds, or a combination of these treatments.
It has been proposed to introduce into the support compounds containing, in the same molecule, both at least one metal from group VI and at least one metal from group VIII. Heteropolyacid salts of metals from group VIII, for example the cobalt and/or nickel salts of phosphomolybdic, phosphotungstic, silicomolybdic and silicotungstic acid, have been used in particular for this purpose. This proposal is described, for example, in documents U.S. Pat. Nos. 2,547,380 and 2,608,534.
The use of reductive compounds to prepare catalysts has already been proposed.
Thus, U.S. Pat. No. 5,332,709 proposes to stabilize aqueous solutions of compounds of the elements from groups VI B and VIII by the simultaneous use of an inorganic phosphorus acid, such as phosphoric acid, and a reductive compound. The solutions contain a high proportion of phosphoric compound, generally from 5 to 15 or 20% by weight of the phosphorus acid.
Similarly, U.S. Pat. No. 5,338,717 proposes to reduce a compound of a metal from group VI, such as phosphomolybdic acid, by using a reducing agent after the compound of the metal from group VI has been deposited on a support and the impregnated support has been dried. The catalyst obtained, which also contains a compound from group VIII, is indicated as being useful as a water-treatment catalyst.
Phosphoric acid is also used in EP-A-0,300,629, EP-A-0,357,295 and EP-A-0,496,592 with compounds of metals from groups VI and VIII and with a sulphur compound capable of sulphurizing the catalyst. In this case also, the phosphoric acid is present in large amount.
Whatever the merit of the catalysts obtained by these methods, studies should be carried out in order to obtain more active catalysts and/or catalysts with longer lifetimes, in order to meet the increasingly stringent environmental legislation requirements.